This invention relates to an induction furnace employing a graphite crucible and more particularly to an improvement in an air-insulated furnace structure.
Hitherto, furnaces of a back-lining type and a coil-cementing type as shown in FIGS. 1 and 2 respectively have been generally known as the induction furnace using the graphite crucible.
The induction furnace of the back-lining type as shown in FIG. 1 is formed of a graphite crucible GC, the outer periphery and the bottom of which is applied with a back-lining BL of a refractory material which is in turn circumferentially mounted with a coil C. The induction furnace of the coil-cementing type as shown in FIG. 2 is constituted in such a way that the graphite crucible GC is surrounded by a coil cement GC which is spaced to provide an insulating space. The coil cement CC is circumferentially mounted with a coil C.
In considering the arrangements, either type of the known induction furnaces back-lining or coil cement of relatively large thickness are required resulting in a lower power factor and efficiency. Further, in the known induction furnace since the back-lining and the coil cement have a high thermal conductivity, the high heat transfer from the graphite crucible to the coil is proportionally enlarged with a disadvantageous high heat dissipation throughout the furnace. In particular, the furnace of the coil-cementing type is subjeced to the extremely high heat dissipation because the insulating space is opened to the outer atmosphere. The back-lining type, on the other hand, has a disadvantage in presenting great difficulty in renewal of the refractory. Furthermore, the conventional induction furnaces using a graphite crucible is not provided with appropriate safety measures for discharging molten metal out of the furnace upon leakage of the molten metal.
As a result of every endeavor to overcome all of the disadvantages hereinbefore described and to obtain an conveniently construction and highly safe induction furnace of high performance, the inventors have provided a novel furnace having many advantages such as convenient operation and maintenance, less heat dissipation, improved electric characteristic and excellent thermal insulation in which the graphite crucible is surrounded by an air space defined by the thermal insulation sheath on which is in turn mounted with a coil. The graphite crucible with its bottom is placed on a furnance bed through an interposed thermal insulating material and the bottom of the air to space is closed. A thermal insulating wedge is releasably mounted on an outer periphery of the upper open end of the graphite crucible in order to completely seal the air space.
The furnace bed for supporting the graphite crucible is preferably provided in which an opening with a detecting sensor is located through which the molten metal is discharged. The opening is ordinarily closed by a blank plate of material which is combustible upon contact with the molten metal so that the molten metal leaked through the air phase may be discharged out of the furnace together with an accurate detection of the leakage of the molten metal by the sensor which ensures an enhancement of the safety of the induction furnace.